1. Field of the Invention
The present invention relates to compass systems which use a fluxgate to measure the orientation of the vessel in which the system is mounted relative to the earth's magnetic field.
A fluxgate operates by the induction of pulses in detection coils by current applied to a drive coil. All the coils may have a common core. Application of an alternating signal to the drive coil induces current pulses in the detection coils, the areas of which pulses is proportional to the component of the magnetic field perpendicular to the detection coil. The earth's magnetic field provides the magnetic flux through the detection coils, in the absence of other fields. Hence if the directions of the detection coils relative to the vessel are known, the pulses from the coils can be used to determine the angle of the earth's magnetic field relative to the detection coils, and hence the orientation of the vessel of that field.
2. Description of the Prior Art
It is known to use a fluxgate as part of a compass system. The standard method of detecting the angle of the fluxgate to the field is to apply an alternating, e.g. sinusoidal, drive signal to the fluxgate and detect the phase of the outputs. FIG. 1 of the accompanying drawings shows the output produced by the fluxgate when an alternating drive signal in the form of a sinusoidal voltage (shown dotted) is applied to it. The area of each large and small pulse L and S respectively is the same and proportional to the field perpendicular to the detection coil. This output is analysed by filtering at twice the frequency of the drive signal and the phase angle between the drive signal and the output signal provides a measure of the angle between the perpendicular to the coil and magnetic north. Due to its filtering, this system is known as a second harmonic fluxgate. It has the disadvantage that active filters are needed for the filtering of the output of the fluxgate and it is essential that the drive signal should be accurately synchronised to prevent phase errors developing. For this reason, the detection circuits are complex and expensive, and pivoting magnet systems have been preferred.